1. Field of the Invention
The present invention relates to a magnetic material detecting apparatus for detecting in a non-contact manner a fine quantity of magnetic material contained in ink for use in printing on, for example, paper sheets or the like.
2. Description of the Related Art
There has been widely known a method for identifying paper sheets or the like by detecting a magnetic material contained in a printing ink for use in printing on the sheets or the like. A differential coil type transformer system has been conventionally known as a method for detecting a magnetic material contained in a printing ink. In this system, a primary coil is wound around a center of an S-shaped core; secondary coils are wound on the side of two openings formed with small gaps, respectively; a paper sheet or the like is allowed to pass over one of the two openings; and thus, a difference in induced voltages generated by the two secondary coils is output. In addition, there has been known a method for defining a small gap at a part of an annular core on which a coil is wounded, and detecting a change in induced voltage of the annular core when a paper sheet or the like passes through the gap.
Jpn. Pat. Appln. KOKAI Publication No. 2002-42203 discloses a magnetic material detecting apparatus comprising a pair of I-shaped cores each having coils wound at both ends in a longitudinal direction, the cores being arranged in a manner facing to each other at one end thereof with a gap. The coils arranged at the ends on the facing side of the pair of cores and the coils arranged at the ends on the other side of the cores are connected in series to each other, respectively, thereby forming the two coils. A magnetic material passing between the cores is detected in a non-contact manner by detecting a difference in induced voltages generated in the two coils.
Jpn. Pat. Appln. KOKAI Publication No. 2004-54911 discloses a magnetic material detecting apparatus comprising I-shaped cores which are arranged in a manner facing to each other at one end thereof with a gap. A primary core is arranged at a center of each core, and secondary coils are arranged at ends on a facing side of the cores and at ends on the other side, respectively. The coils arranged at the ends on the facing side are connected to in series each other, and the coils arranged at the ends on the other side are connected in series to each other. A differential arithmetic circuit produces a magnetic signal in response to a difference in coil signal at both of the ends. Consequently, it is possible to reduce an adverse influence between adjacent cores in a plurality of pairs of core arrays.
In the above-described magnetic material detecting apparatuses, paper sheets or the like are allowed to pass through the gap defined between the magnetic cores arranged opposing to each other, thereby detecting a magnetic material. With the above-described configuration, the detection sensitivity to the magnetic material is inconstantly varied in accordance with a position in the gap through which the sheets or the like pass. That is, the detection sensitivity is stable in the vicinity of the center of the gap, and in contrast, the detection sensitivity becomes higher as the sheets or the like approach one of the magnetic cores.
In the magnetic material detecting apparatuses disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 2002-42203 and 2004-54911, a signal output generated when the sheet or the like passes near an end surface of the magnetic core has tended to become greater by about 50% than a signal output generated when the sheet or the like passes the center of the gap.